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Abstract
Shale wellbore instability is a complex difficulty encountered during drilling all over the world. And the most important factor to determine shale wellbore stability is the distribution and expansion of micro-cracks in hard brittle shale. The usual ways to avoid collapse of shale wellbore mainly include increasing the inhibition of drilling fluid filtrate, improving the quality of drilling fluid cake and sealing the pores and micro-cracks near the wellbore surface. However, the shale wellbore collapsing difficulty was not solved effectively in field, which mainly because that the effective sealing is difficult to achieved in the case of crack width is unknown. Furthermore the drilling fluid cake is more difficult to be formed on shale sections because of the filtration rate is very low compared to sand sections under the same differential pressure.

A kind of nanomaterial has been applied to achieve a sealing film rapidly, improve the sealing strength of the cake and reduce the mud cake permeability, which have been verified through the use of sound wave propagation speed, the pressure transmission experiment and cake strength. The technology had also been used in Well Exp1, which exhibited a smooth borehole and the borehole enlargement rate was below the wells in the same block.

Introduction
When the sand formation is drilled, the drilling fluid will filtrate into formation pores under overbalanced differential pressure. Then after a while formation pores will be plugged by solid particles gradually and drilling fluid cake will be formed so the filtration will be stopped. The mud cake forming with ultra-low filtration requires the match between the pore throat size and the shielding/ plugging particle size. Abrams (1977) proposed that in order to form bridging, particle sizes should not be larger than one third of the pore throat size. It is important to form a dense drilling fluid cake for shale formation because the cake will prevent drilling fluid filtrate invasion to reduce the hydration and sloughing. However, a mud cake may not be formed for shale formations because conventional drilling fluid particle sizes are usually much larger than the shale pore throat size. The advancements in the science of Nanotechnologies can play a major role in solving this problem. Nanotechnology represents the development and application of materials, methods, and devices in which critical length scale is on the order of 1~100 nm and where critical functionality is not a direct manifestation of the atomic or macroscale properties. There are numerous areas in which nanotechnology can contribute to more-efficient, less-expensive, and more-environmentally sound technologies than those that are readily available. The applications of nanomaterial in drilling fluids are mainly to prevent common problems like borehole wall stability, the thermal stability and lubricity. The dense drilling fluid cake will be formed through the adding of nanomaterial to keep borehole wall stable.

Shale properties
Shale Permeability and Porosity. Shale samples used in this study were prepared from shale cores of the Sha3 formation recovered from Well A. Shale samples permeability and porosity were determined in lab and the results were showed in Table 1, which are much lower than sandstones. It means that the filtration rate of shale sections is very low compared to sand sections under the same differential pressure and the drilling fluid cake is more difficult to be formed on shale sections.